1. Field of the Invention
The present invention generally relates to a furnace for performing heating and/or heat treatment of a work, and particularly relates to a furnace provided with a plurality of vertically-disposed furnace chambers each having at least one disc-like turntable, the turntables of the respective furnace chambers being disposed coaxially, the furnace chambers being made different in temperature from each other.
2. Description of the Prior Art
In the process of producing a work such as a gear, a shaft, a piston pin, a crank shaft, or the like from steel products, for example, Ni--Cr steel, Ni--Cr--Mo steel, Cr steel, Cr--Mo steel, or the like, the steel products are subject to heat treatment to strengthen the metal structure thereof after worked through forging. Examples of the heat treatment includes carburizing. In the process of carburizing, even if a CO gas suitable for performing carburizing is generated, the carburizing phenomenon cannot be generated if the steel product forming a work is ready to receive carbon. That is, steel does not have capability of receiving carbon before the steel has become .gamma. iron which can exist only at a temperature not lower than the A.sub.1 transformation temperature of about 723.degree.C. In order to perform carburizing, therefore, it is necessary to heat steel to a temperature not lower than 723.degree.C. Steel exists in the state of .alpha. iron at a temperature lower than 723.degree.C., and the capability of performing carburizing of such an .alpha. iron is extremely small. Accordingly, the carburizing temperature is generally set to a value within a range of from 880.degree.C. to 930.degree.C. Namely the carburizing temperature is selected to be higher than the above range, so that a deep carburized layer can be obtained in short time. This is because carbon atoms which have entered the steel easily enter the inside of the steel, and this phenomenon is called diffusion. A carburized work is further heated to a suitable quenching temperature, for example, within a range of from 800.degree.C. to 850.degree.C., and then cooled rapidly in oil so as to be hardened.
The foregoing carburizing and diffusion has been performed by a process in which a furnace system having horizontally arranged several furnaces or batch furnaces is prepared and a tray or a basket in which a plurality of work is held is passed through the batch furnaces from the inlet of the system to the outlet of the same so that the works are subject to carburizing, diffusion, and quenching and then taken out from the outlet of the system. Since a plurality of works is held in a tray, a basket, or the like, the heat to be applied to the works held at the center portion of the bottom of the tray or the like is reduced because it is absorbed by the works held at an upper portion or near the outer periphery of the tray or the like, while the heat applied to the latter works is proper. Accordingly, an agitating fan has been provided to make the furnace temperature even. It is however difficult to make the furnace temperature even only by providing a fan, and therefore a plurality of heating means is additionally provided. Further, thermal distortion is caused not only in works but in jigs or the like supporting the works, because also such jigs or the like are heated to the carburizing temperature and the diffusion temperature. Further, cooling distortion is also caused in the jigs or the like in cooling operation. It is therefore impossible to use the jigs or the like for a long time and the jigs or the like are regarded as consumption goods. Furthermore, when the atmospheric temperature in the furnaces is controlled in performing heat treatment on the work, it is necessary to take the heat capacity of the trays, baskets, or jigs or the like into consideration. However, the heat capacity of a jig or the like per se is generally larger than that of a work and therefore the quantity of heat required for heating is increased so that uneconomical operation is obliged to be carried out.
The inventor of this application has proposed a furnace in which a plurality of turntables is vertically arranged in the inside of a cylindrical furnace chamber having a vertical axial line, works are mounted on the turntables, and the turntables are horizontally rotated to make the works subject to heat treatment, as disclosed in Japanese Patent Post-examination Publication No. 61-544 published on Jan. 9, 1986 (filed on Aug. 27, 1982, and granted as Japanese Pat. No. 1,339,640) and in Japanese Patent Post-examination Publication No. 62-48152 published on Oct. 12, 1987 (filed on Oct. 11, 1984, and granted as Japanese Pat. No. 1,441,721). More specifically, the furnace is provided with a cylindrical furnace chamber oriented so that its axis is directed in the vertical direction. A plurality of turntables is coaxially disposed so as to be horizontally rotatable and a plurality of work mounts for holding works thereon is provided at suitable intervals on an outer circumference of an upper surface of each of the turntables. In the central portion of the turntables, a circulating fan which rotates at a speed higher than that of the turntables is provided so as to make the temperature in the furnace uniform and diffuser blades are concentrically radially provided so as to be in opposition to the circulating fan. Further, a work insertion hole and a work take-out hole are provided in side portions of a furnace wall so that works can be mounted on the work mounts through the work insertion hole by means of a fork and works after treated can be taken out of the take-out hole by using the fork. In this, the furnace can be made relatively compact as a whole, and a large number of works can be successively continuously subject to heat treatment. Further, the heating air current from heating means is diffused by the diffuser blades through the relative rotation between the turntables and the circulating fan, so that heat transmission of works can be rapidly performed.